Cross-module association datasource writes null DestinationEntityId, crashes Studio Pro (and mxbuild's own project loader) on open

[MendixMau]

Summary

A datagrid/dataview/listview widget whose datasource traverses a cross-module
association ($currentObject/OtherModule.Assoc, where the association is declared in a
different module than the one being edited) is written with an empty/null DestinationEntity
on the underlying DomainModels$EntityRefStep BSON. mxcli exec reports success. Any
subsequent load of the resulting .mpr — by Studio Pro opening the project, or by mx/mxbuild's
own project-load step — throws ArgumentNullException from
EntityRefStep.set_DestinationEntityId and fails hard. The project becomes unopenable.

Same-module association traversals are unaffected; the defect is isolated to associations that
cross a module boundary.

Environment

• mxcli: v0.16.0 (2026-07-12T11:44:17Z) — the current tagged Latest release
• Mendix Studio Pro / mxbuild: 11.12.0 Beta (also reproduced previously on 11.12.1 in a real
  conversion project, 2026-07-13)
• OS: macOS (Darwin 25.5.0), arm64

Reproduction

create module "BUG20A";
/
create module "BUG20B";
/
create persistent entity "BUG20A"."EntityA" (
  "Name": string
);
/
create persistent entity "BUG20B"."EntityB" (
  "SomeAttr": string
);
/
create association "BUG20A"."Assoc_AToB"
from "BUG20A"."EntityA" to "BUG20B"."EntityB"
type reference;
/
create page "BUG20A"."Page_EntityA" (
  params: { $Obj: "BUG20A"."EntityA" },
  title: 'EntityA Page',
  layout: Atlas_Core.Atlas_Default
) {
  dataview dv1 (datasource: $Obj) {
    datagrid dgItems (
      datasource: $currentObject/"BUG20A"."Assoc_AToB"
    ) {
      column colAttr (attribute: SomeAttr, caption: 'Some Attr')
    }
  }
}
/

./mxcli check bug20-repro.mdl                    # => Syntax OK (6 statements), Check passed!
./mxcli exec bug20-repro.mdl -p EmptyTest.mpr    # => reports success on every statement:
#   Created module: BUG20A
#   Created module: BUG20B
#   Created entity: BUG20A.EntityA
#   Created entity: BUG20B.EntityB
#   Created association: BUG20A.Assoc_AToB
#   Created page BUG20A.Page_EntityA

./mxcli docker check -p EmptyTest.mpr
#   Using mx: /Applications/Mendix Studio Pro 11.12.0 Beta.app/Contents/modeler/mx
#   Updating widget definitions in EmptyTest.mpr...
#   ERROR: System.AggregateException: One or more errors occurred. (An error occurred when
#   trying to set the 'DestinationEntity' property of a Entity ref step in a Page with ID
#   58d21118-e0ad-49d5-b9bc-1071848a5276.)
#    ---> System.InvalidOperationException: An error occurred when trying to set the
#   'DestinationEntity' property of a Entity ref step in a Page with ID
#   58d21118-e0ad-49d5-b9bc-1071848a5276.
#    ---> System.ArgumentNullException: Value cannot be null. (Parameter 'value')
#      at Mendix.Modeler.DomainModels.Refs.EntityRefStep.set_DestinationEntityId(EntityIdentifier value)
#      ...
#   Error: project check failed: exit status 1

Independently decoding the raw BSON of the affected page unit
(mprcontents/58/d2/58d21118-e0ad-49d5-b9bc-1071848a5276.mxunit, the exact unit ID named in the
crash) with bson.decode() (Python pymongo) shows the field directly, rather than relying on
the crash message or a describe/read-back command:

{
  "$Type": "DomainModels$EntityRefStep",
  "Association": "BUG20A.Assoc_AToB",
  "DestinationEntity": ""
}

Association correctly resolves; DestinationEntity is an empty string where a fully-qualified
entity identifier (BUG20B.EntityB) is expected.

Opening the same .mpr in a fresh Studio Pro 11.12.0 Beta instance reproduces the crash visually:
Studio Pro shows an "Error" dialog window and the process then terminates.

Expected behavior

The datagrid/dataview loads objects across the association from the current context object; the
.mpr remains loadable by both mx check/mxbuild and Studio Pro.

Actual behavior

• mxcli exec reports success with no warning.
• mxcli check --references / mx check do not reliably surface a build error for this
  defect — in this retest, mxcli docker check (which invokes Studio Pro's own bundled mx
  tool) crashed outright with the same exception during its own project-load step, rather than
  returning a clean error count; in the original 2026-07-13 finding on a real conversion project,
  the same construct produced a clean "0 errors" from the build gate, with Studio Pro crashing
  separately on its own subsequent project open. Either way, no build-time error is ever
  reported for the actual defect — the tooling either crashes before it can count errors, or
  passes clean and defers the crash to Studio Pro.
• Studio Pro cannot open the resulting project at all: ArgumentNullException at
  EntityRefStep.set_DestinationEntityId, project load aborts.

Root cause (inferred)

mxcli's page/widget writer does not resolve the destination entity's module-qualified identifier
when constructing the DomainModels$EntityRefStep for a datasource expression that traverses an
association declared in a different module than the widget's own context ($currentObject/ OtherModule.Assoc). The DestinationEntity field is written as an empty string instead of the
resolved <Module>.<Entity> identifier. Same-module traversals resolve correctly, which is
consistent with the writer looking up the destination entity relative to the wrong (or no)
module context specifically for the cross-module case.

Severity

Critical, for Mendix 11.12.x (the version tested here and in the original 2026-07-06/07-13
findings) — the defect makes the entire .mpr unopenable in Studio Pro, with no warning at
mxcli exec time and no reliable build-time error to catch it before that point.

Workaround

Write the page via MDL without the cross-module association datasource widget, exec and verify
Studio Pro opens cleanly, then add the cross-module datasource widget via mxcli --mcp's
pg_patch_page while Studio Pro is open (that write path does not go through the same
buggy writer).